1. Field of the Invention
The present invention relates to a porous member which may be used as a static pressure type pneumatic bearing, filter, humidity sensor or the like, and also pertains to a method of manufacturing such porous members.
2. Description of the Related Art
Heretofore, there has been used a static pressure type pneumatic bearing which is adapted to form a pneumatic film on its surface by ejecting gas therefrom, so as to guide a magnetic tape or the like moving object under an essentially contact-free condition, or to reduce the friction between the bearing and a rotational shaft journalled thereby.
It is a conventional practice to provide such a static pressure type pneumatic bearing with an orifice restriction, autogenous restriction, surface restriction, porous restriction or the like, for improving the load-bearing performance of the bearing. Among these restrictions, it has been confirmed that the porous restriction serves to effectively realize a bearing having a distinguished load-bearing performance and a high bearing stiffness. However, the operation or function of the porous restriction tends to become unstable due to a pneumatic hammer phenomena.
Thus, Japanese Laid-open Patent, Publication Nos. 53-56,447 and 59-166,699 each discloses a porous member having a porous restriction provided on its surface with further restrictions. Specifically, the technology disclosed in Japanese Laid-open Patent Publication No. 53-56,447 is to form a coated layer on the pore surface of the porous body by ion plating, ordinary plating, deposition, coating or the like. Further, the technology disclosed in Japanese Laid-open Patent Publication No. 59-166,699 is to form burrs on the bearing surface side of the porous body, and subsequently cause the burrs to undergo a partial decomposition by an electrolytic treatment so as to control the cross flow rate, or gas flow rate. However, these known measures suffer from various drawbacks is that they are applicable only to porous members of a relatively simple configuration, and it is still difficult to properly control the cross flow rate even in the case of porous members having a simple configuration, thereby necessitating a series of adjustments on a trial-and-error basis.
Moreover, the above-mentioned various treatments have to be performed with respect to the surface of the porous body during or after a precise machining. Consequently, there may be instances wherein a sufficient surface accuracy cannot be achieved without carrying out an additional surface finishing. Besides, the required treatments have to be performed with an elaborate facility or large scale equipment which is not only disadvantageous in terms of cost, but also in that it may give rise to such problems as pollution due to a waste treatment to be performed with respect to the plating liquid or electrolyte.
It is therefore a primary object of the present invention to provide an improved porous body which can be manufactured in such a manner which is capable of eliminating or mitigating the drawbacks of the conventional structures.
Another object of the present invention is to provide an improved method of manufacturing porous bodies, which is capable of eliminating or mitigating the drawbacks of the conventional structures.
According to one aspect of the present invention, there is provided a porous member comprising a porous body having an outer surface portion and a number of internal pores, said outer surface portion of the porous body being covered by a surface layer comprised of fine particles which are impregnated in the outer surface portion of the porous body and which have been subjected to such a heat treatment as to form restricted passages which are in communication with, and smaller in size than said pores.
With the above-mentioned features of the porous member according to the present invention, the restricted passages which are in communication with, and smaller in size than the pores in the porous body can be formed in a facilitated manner, by forming a surface layer comprised of fine particles which are impregnated in the outer surface portion of the porous body, and subjecting the fine particles to a heat treatment and causing adhesion of the fine particles with each other.
The porous body may comprise a ceramic body having a mean pore diameter which is on the order of 10 xcexcm. The fine particles may comprise an inorganic oxide having a mean diameter which is on the order of 0.01-1 xcexcm.
According to another aspect of the present invention, there is provided a method of manufacturing porous members, comprising the steps of: forming a surface layer on an outer surface portion of a porous body having internal pores, said surface layer being comprised of fine particles which are impregnated in the outer surface portion of the porous body; and heat-treating the surface layer and thereby forming restricted passages which are in communication with, and smaller in size than said pores.
Advantageously, the surface layer is formed by applying to the outer surface portion of the porous body a treating or mixture liquid containing said fine particles, and subsequently drying and solidifying the treating liquid.
In one preferred embodiment of the method according to the present invention, the treating liquid comprises a silica sol in which fine silica particles are dispersed, with the heat treatment performed at a temperature of approximately 400xc2x0 C.
In this instance, the porous body of ceramics or the like impregnated by the dispersion liquid of the fine particles has its pores which are filled by the particles. By subsequently performing a heat treatment with respect to such a porous body, however, the dispersion liquid is dried up and caused to solidify thereby forming cracks as gas passages. The size of such cracks is predominantly dependent upon the concentration of the dispersion liquid, but not on the impregnating time.
Therefore, the fine particles of the surface layer are filled in the pores within the porous body and cracks are then formed in such surface layer as passages for conducting fluid therethrough. Thus, the method of the present invention is suitable for controlling the gas flow rate across the porous body having a relatively complex configuration. Since the size of the cracks influential on the gas flow rate is predominantly dependent upon the concentration of the dispersion liquid, the gas flow rate provided by the porous body can be controlled in a very facilitated manner and the desired flow rate can be achieved by a single treatment step. Moreover, the outer surface of the porous body does not require any plastic deformation, such as formation of burrs or the like. It is therefore possible to preserve the desired accuracy of the finished surface of the porous body as it is, without requiring any after-treatment or facilities therefor. Furthermore, the required steps are only to impregnate the porous body with the sol liquid and perform a heat treatment; thus, an elaborate facility is unnecessary and the porous body can be advantageously manufactured at a low cost, without giving rise to any environmental problems.
In another preferred embodiment of the method according to the present invention, the treating liquid is a mixture comprising a resin emulsion and the fine particles dispersed in the emulsion. The resin emulsion may contain a thickener. In this instance, when the porous body is impregnated by a treating liquid comprised of a resin emulsion and fine particles, and the impregnated mixture (or treating) liquid is then dried and solidified, the pores in the outer surface portion of the porous body are clogged with the fine particles. By subsequently performing a heat treatment, however, the fine particles are adhered to each other while the resin emulsion is decomposed and removed thereby forming restricted passages which are capable of conducting gas or absorbing liquid.
This embodiment, too, is suitable for controlling the gas flow rate and the like of the porous body having a relatively complex configuration. The size of the restricted passages is influential on the gas flow rate or liquid absorption amount, and is dependent predominantly on the proportion of the resin emulsion within the treating liquid, but essentially not on other factors, Therefore, the size of the restricted passages can be controlled in a facilitated manner, and the desired flow rate can be achieved by a single treatment step. Moreover, the surface layer of the fine particles formed by an adhesion induced by the heat treatment has an excellent mechanical strength. In this instance also, as mentioned hereinbefore, it is possible to preserve the desired accuracy of the finished surface of the porous body as it is, without requiring any after treatment or facilities therefor. The required steps are only to impregnate the porous body with a liquid mixture the treating liquid and perform a heat treatment; thus, an elaborate facility is unnecessary and the porous body can be advantageously manufactured at a low cost, without giving rise to any environmental problems.
Preferably, the porous body comprises alumina and the treating liquid is a mixture comprising a resin emulsion of a polyvinyl acetate group and fine silica particles dispersed in the emulsion. In this instance, preferably, the heat-treatment of the surface layer is performed at a temperature which is not higher than approximately 700xc2x0 C.
In this instance, the treating liquid may be a mixture comprising a resin emulsion, first fine particles having a relatively low melting temperature, and second fine particles having a relatively high melting temperature, said first and second fine particles being dispersed in the emulsion. Then, the heat treatment of the surface layer may be performed at a temperature which is higher than the melting temperature of the first fine particles so that the second fine particles are connected with each other by the first fine particles.
When the porous body comprises alumina and the treating liquid is a mixture of an emulsion of a polyvinyl acetate group resin containing a thickener and fine particles of silica having different mean diameters and dispersed in the emulsion, the heat treatment can be performed at a temperature of approximately 400xc2x0 C.